Advanced semiconductor technology introduces many complex structures that need to be measured in order to monitor and control the structures and the semiconductor manufacturing processes that make the structures, and adjust these processes to reduce defects and promote higher yield. As structure complexity increases, more parameters are needed to properly characterize the structures in OCD (Optical Critical Dimension) models. Unfortunately, having too many parameters in an OCD model can lead to undesirable correlations between the parameters and/or low sensitivity to changes in the parameters, often resulting in unacceptable OCD metrology measurement error. Methods for reducing the number of these parameters without worsening such measurement error would be advantageous.